The insulin-like growth factor-I receptor (IGF-IR) mediates the growth-promoting and differentitive effects of the IGFs whereas the insulin receptor (IR) mediates the metabolic responses to insulin. To determine the different signaling pathways involved in these two similar receptor systems we have analyzed a number of unique substrates including pp120 the ecto-ATPase and the Crk proto-oncogene family of proteins. pp120, the liver-specific ectoATPase, is a substrate for the IR and is rapidly phosphorylated on tyrosines following activation of the IR. It is involved in IR internalization since co-transfection of IR and pp120 enhances the internalization rates of the IR. No effect is seen following IGF-IR activation. In cells and in vitro assays pp120 associates with the IR via its C-terminal domain. When the C-terminal domain of the IR is swapped with the equivalent region in the IGF-IR, the IGF-IR is then able to associate with and phosphorylate pp120. Thus, while the exact function of pp120 in insulin action is as yet not defined it is apparently an IR-specific substrate, interacting via the divergent C-terminal domain of the IR. The Crk family of pro-oncogenes, Crk-II and CrkL, are phosphorylated on tyrosines by activated IGF-IRs and demonstrate much less involvement in the IR signaling pathways. Crk-II interacts with the IGF-IR in vitro, whereas CrkL does not. Both proteins interact with IRS-4 a newly discovered 180 phosphoprotein of 180 kDa following IGF-IR activation. Crk-II interacts via its SH2 domain, whereas CrkL demonstrates much higher affinity because of the use of both SH2 and an SH3 domain for this interaction. In cells overexpressing these adapter proteins, different biological responses were seen. CrkL caused a transforming phenotype, whereas Crk-II was anti-oncogenic. Thus, two very similar members of a proto-oncogene family are substrates for the IGF-IR, however, they have different biological effects and different structural interactions with the IGF-IR and IRS-4 molecules which suggests interaction with different downstream signaling pathways.